Add unified spacing utils

[yurii-vasyliev]

Done

• Added common spacing classes to be used between elements.

Fixes WD-2884

QA

• Open demo
• Check the new example page
• Review updated documentation

Check if PR is ready for release

If this PR contains Vanilla SCSS code changes, it should contain the following changes to make sure it's ready for the release:

• PR should have one of the following labels to automatically categorise it in release notes:
  • Feature 🎁, Breaking Change 💣, Bug 🐛, Documentation 📝, Maintenance 🔨.
• Vanilla version in package.json should be updated relative to the most recent release, following semver convention:
  • if CSS class names are not changed it can be bugfix relesase (x.x.X)
  • if CSS class names are changed/added/removed it should be minor version (x.X.0)
  • see the wiki for more details
• Any changes to component class names (new patterns, variants, removed or added features) should be listed on the what's new page.

[webteam-app]

Demo starting at https://vanilla-framework-4717.demos.haus

[bartaz]

Looks good, thanks! Mostly some cosmetic comments.

[bartaz]

Could you also add an entry in releases.yaml file: https://github.com/canonical/vanilla-framework/blob/main/releases.yml ?

It should reference the part that changed: "Vertical spacing", "New" - because we add new class names, and link to the updated docs page.